As shown in FIG. 25, there is conventionally a spark plug 9 for an internal-combustion engine used as an ignition means of the fuel-air mixture introduced into a burner of internal combustion engine, such as a car (for example, see Patent Document 1).
The spark plug 9 has a center electrode 94 and an earth electrode 95.
The earth electrode 95 is fixed to an attachment fitting 92 and has a projection part 951. The projection part 951 is attached to the earth electrode 95's opposed surfaces which oppose the center electrode 94, so that the projection part 951 is provided opposite to the center electrode 94.
However, there is a following problem in the spark plug 9. That is, in the spark plug 9, since the projection part 951 is formed by attaching another component to the earth electrode 95, the man-hour in the manufacturing process of the spark plug 9 will increase. Consequently, there is a possibility that it may become difficult to raise the productivity of the spark plug 9. Moreover, in forming the projection part 951 with the precious metals etc., there is a possibility that material cost may become high.
On the other hand, as shown in FIG. 26, There is a spark plug 90 whose projection part 951 with a convex curved shape is integrally shaped with a flat earth electrode 95 by giving bending processing etc. to the flat earth electrode 95 (for example, see Patent Document 2). In the spark plug 90, in order to secure the amount of projection of the projection part 951, it is necessary to enlarge the depth of a concave part 952.
However, when the depth of the concave part 952 is enlarged, there is a possibility that the path of the earth electrode 95 for heat dissipation may become long. Consequently, heat dissipation of the earth electrode 95 cannot fully be performed, there is a possibility that it may become difficult to obtain the spark plug 90 excellent in heat resistance.
[Patent Document 1]    Japanese Patent Application Laid-Open No. 2003-317896
[Patent Document 2]    Japanese Patent Application Laid-Open No. S52-36238